In routine spin-echo or fast spin echo (FSE) magnetic resonance (MR) imaging, tissue contrast is governed by the echo time (TE). By an appropriate choice of TE, the contrast between native tissue and pathology may in theory be optimized. This optimization is typically governed by the relative T2 values of the two species. In practice, a range of T2 values may be present in both native tissue and pathology (references 1, 2). Moreover, multiple tissue types as well as multiple pathologies, all with potentially different T2 values, may be present (references 3-6). As a consequence, any single TE is likely suboptimal for the visualization of at least some of the tissue/pathology. Additional scans at different TEs may need to be performed. However, time constraints limit the number of TEs that may be acquired. More importantly, the T2 values present, and therefore the optimal TEs, are not necessarily known a priori.
In theory, a post-acquisition technique which would allow a user to selectively vary the imaging TE dynamically through a range of TE values, analogous to “windowing and leveling” the grey-scale image display range of digital acquisitions, could provide a new means of diagnostic interpretation of clinical musculoskeletal MR imaging. Such a technique could allow for interpretation of images at any selected TE, but would also allow for visual evaluation of the dynamic variation of T2 contrast within tissues by continuous real-time user variation of the image TE settings.
An imaging strategy to accomplish this is to first generate a T2 map of the anatomy. This provides quantitative T2 and proton density (PD) values for every pixel in the image. Using a model for TE decay, for example a mono-exponential model, a synthetic image Isyn(x,y) at any TE value (i.e. from 0 to infinity) comprised of pixels at location (x,y) may then be generated via the following formula (here, using a mono-exponential model):Isyn(x,y)=PD(x,y)·e−TE/T2(x,y)  (equation 1)
The principles of this synthetic-TE technique were first described in 1984 (reference 7). However, early implementations of this approach were limited by hardware constraints, as well as inadequate tools for proper radiological analysis of the synthetic-TE images (references 7-17).
There drawbacks with existing synthetic-TE approaches (references 7-13, 17). One is lack of appropriate software for a proper, formal radiological analysis of the synthetic-TE images. This limited the effectiveness, and the ultimate utility of early synthetic TE techniques. Another drawback is that existing techniques do not provide any indication as to how reliably the “synthetic” images represent the underlying “true” anatomy. For example, the synthetic image may be considered unreliable if the decay model used to generate the synthetic image has a poor-quality fit with the actual MR signal behavior. For example, one key assumption of previous synthetic-TE techniques is that the T2-decay data is described accurately by a mono-exponential decay model. In reality, the possibility of deviation from this model is not negligible, as many factors are known to affect the nature of T2 decay (e.g. diffusion, exchange between tissue compartments, the presence of more than one T2 species within a pixel, etc). If the MR signal is not described accurately by a mono-exponential decay model, then the synthesized-TE images will provide unreliable information. Existing synthetic-TE approaches have no means of assessing the presence of this possible unreliability.
These drawbacks and other limitations of existing synthetic-TE approaches have impeded the use of synthetic-TE techniques on a routine basis.